1. Field of the Invention
The present invention relates to an electron emission element and a method of manufacturing the electron emission element, and also to a display device using the electron emission element and a method of manufacturing the display device. The present invention is applicable to an image display device, an electron beam exposing device, etc.
2. Description of the Related Art
Application of high electric field of about 107 (V/cm) level to the surface of metal or semiconductor induces such a phenomenon that electrons are emitted from the surface of metal or semiconductor into vacuum, and this phenomenon is called as xe2x80x9cfield emissionxe2x80x9d. The field emission is caused by tunneling of electrons in the vicinity of the Fermi energy level in metal or electrons excited up to the conduction electron band into the vacuum level. However, in the case of the semiconductor, electrons located in the valence band or various levels existing between the bands, such as the impurity levels, the surface levels, etc. may be emitted.
A field emission type cold cathode has such a merit that the electron emission current density can be set to a larger value as compared with that of a thermionic cathode. In the case of thermionic cathodes, the field emission density is limited to about several tens of amperes per one square centimeter at maximum. On the other hand, with cold cathodes, the electron emission current density of about 107 to 109 amperes per one square centimeter can be achieved. Therefore, use of the field emission type cold cathode is particularly effective to design micron-sized miniature vacuum electron devices.
An actual example of a vacuum micro-electric device using the cold cathode, was first reported by Shoulders in 1961(Adv.Comput.2(1961)135), and he reported a method of manufacturing a 0.1-micron size device and a minute field emission type diode by using the above device. Further, Spindt, et al. reported fabrication of an array structure in which a number of micron-size cold cathodes (triodes) having gates formed on a single substrate by a thin film technique (J. Appl. Phys. 39 (1968) 3504). Following this year, various reports have been submitted.
Various types of structures have been proposed for the vacuum micro-electronic device. According to the report of Spindt, et al., there is proposed a structure having a micron-size minute conical emitter having a sharp tip and a electron extracting electrode (gate) having an open portion located just above the emitter. An anode is provided above the emitter.
With such a structure, the electric field is concentrated on the tip portion of the emitter, and the current of electrons emitted from the emitter to the anode can be controlled by the voltage applied across the gate and the emitter.
As other examples of devices having the same structure, there have been reported various reports for manufacturing the devices having the same structure by using a method using anisotropic etching of Si (Grey""s method), a mold method using a mold or the like. The common features of the conventional electron emission elements having the above structure resides in that each of these structures has an extremely sharp emitter tip portion, the radius of curvature of which is equal to about several nanometers, and that the electric field applied at the tip by the difference between the gate voltage and the emitter voltage is increased 100 times to about 1000 times as compared to the voltage difference divided by the gate-emitter distance, resulting from the effect of the concentration of the electric field at the sharply pointed tip of the emitter.
The diameter of the opening portion of the gate ranges from the micron order to the sub-micron order. An actual manufacturing process of these elements need to position the gate and the conical emitter inside the minute opening portion. It is technically and economically difficult to perform such a precise positioning work by using lithography. This difficulty can be avoided by using self-alignment techniques. However, use of such techniques rather causes lots of restrictions.
For example, a manufacturing process based on Spindt""s method will be described.
First, after a gate opening is provided, a peeling layer is formed on the top surface of the gate while the film thereof is prevented from being deposited inside the gate. Subsequently, an emitter material is vapor-deposited from the vertical direction. At this time, the opening diameter of the gate becomes gradually smaller due to the increase of the emitter material adhering to the edge of the opening portion of the gate, so that a conical emitter is formed inside the gate opening. Thereafter, the emitter material adhering to the opening portion of the gate is removed by removing the peeling layer.
As reported in J. Vac. Sci. Technol. B13(1995) 487, a conical shape having an ideal ratio (diameter of bottom surface: height) (aspect ratio) can be formed when Mo is used; however, it cannot be formed when Ti or Zr is used. That is, the material usable for the emitter is limited to special materials not only in consideration of the physical properties which directly affect the field emission characteristics, but also in terms of the shaping of the elements. Accordingly, the emitter material is substantially limited to Mo due to a requirement for forming a conical body having an excellent shape in the vapor-deposition process.
Likewise, the emitter material is limited to Si in the gray method because the tip of the Si conical body is sharpened by thermal oxidation in the Gray""s method.
These methods are too low in flexibility to reduce the cost by reconsidering the process and the material.
In order to widen the range of the materials usable for the emitter, it is required to loosen the restriction caused by the manufacturing process, and the following method is known to satisfy this requirement.
This method directs to such an approach that an emitter having a single emission point is not necessarily located at the center portion of the gate, and but a plurality of emission points are provided in the opening portion of the gate, thereby omitting the positioning work between the gate and emitter. Even when this approach is used, the electron emission amount is actually prevented from being remarkably lowered although the loss of the effective current due to withdrawal of electrons emitted from the emitter by the gate is increased.
In general, there are two factors influencing the intensity of electric field at the tip of the emitter. The one is the sharpness of the tip of the emitter and the other is the distance between the gate and the tip of the emitter. Since the electric-field intensity is more greatly dependent on the sharpness of the tip of the emitter, the above approach can be effectively used. Accordingly, this approach makes it easier technically and economically to form a large-area array of electron emission elements. Such an approach is classified into two types.
One type of approach relates to a method of providing an electric-field concentration structure. For example, Japanese Laid-open Patent Application No. Hei-8-329823 discloses such a structure that an infinite number of columnar crystals of beta type tungsten are grown in the opening portion of the gate and electrons are emitted from the pointed portions of the respective crystals.
The other type of approach uses materials having small work function or small electron affinity. This method enables electron emission from a film having no discrete pointed portions. In general, as the work function or the electron affinity is reduced, the field emission is more likely to occur. Semiconductor materials having a broad band gap of about 5 electron volts or more can be used as materials having especially excellent characteristics for such a film. For example, as these materials are known diamond, boron nitride of cubic or hexagonal system, lithium fluoride, calcium fluoride or the like which have extremely low electron affinities.
For these materials, it has been confirmed or suggested that generally, the lowest energy levels of the conduction bands of these materials are lower than the vacuum level compared to the energy state of electrons under vacuum; however, they are nearly equal to the vacuum level within the range from 0.1 to 0.5 eV, or even higher than the vacuum level in some crystal face directions. These materials are called as xe2x80x9cnegative electron affinity (NEA) materialsxe2x80x9d or xe2x80x9cquasi negative electron affinity materialsxe2x80x9d (for example, J. Vac. Sci. Technol. B13(1997)1733).
Each of these materials has such a property that electrons are emitted to the vacuum without strong electric field at the interface between the material and the vacuum because of its negative electron affinity (NEA) property. This effect is realized by forming a conduction passage based on doping, defect/hydrogen termination or the like on the surface of the material or in the bulk thereof and then injecting electrons into the conduction band.
There has been also reported an experimental result suggesting that electric-field electron emission occurs from a conductive microstructure formed in the bulk or on the surface (for example, Science 282(1998)1471). However, in this case, the electron emission does not necessarily occur from the conduction band unlike the electron emission based on NEA. But the electron emission occurs from local levels due to defects or the like existing between the bands or from the valence band. Therefore, the electron emission are not necessarily induced by a mechanism which positively utilizes small electron affinity.
However, most of these materials have excellent characteristics in surface chemical stability and thermal conductivity, and thus the field emission characteristic thereof is less sensitive to the variation of the surface state and thus more stable as compared with the field emission from the metal surface of Mo or the like.
An electron emission element using a projecting structure of metal material does not stably operate under a normal condition unless it is kept under an atmosphere of 10xe2x88x927 torr or less because its characteristic is very sensitive to the surface state. On the other hand, it has been suggested that an electron emission element using diamond or boron nitride can stably operate even under a low vacuum condition of about 10xe2x88x925 torr (J. Vac. Sci. Technol. B16(1998)1207).
Two methods, a film formation method using vacuum deposition and a method using fine particles of NEA material are known for manufacturing an electron emission element using the above NEA material/quasi NEA material (hereinafter collectively referred to as xe2x80x9cNEA materialxe2x80x9d).
Various methods such as a plasma CVD method, a hot filament CVD method, a filtered cathode arc method (FCVAD), a laser application method, etc. have been reported as the vacuum deposition method for diamond, boron nitride of cubic system which are representative NEA materials. The films produced by these methods exhibit polycrystalline structure but, however, are relatively excellent in local uniformity in crystal grain.
Conversely, when the electron emission element is applied to an electron source used for a large-size electron exciting type flat panel display (FED), a large-size film forming apparatus, typically a vacuum chamber, is needed and this causes increase of the cost. This is because the size of the film which can be formed is limited by the size of the film forming apparatus.
Further, the vapor-deposited film of diamond or the like has a large in-film stress and thus it is liable to be peeled off after the film forming process, which induces a practical problem.
These problems can be avoided by using minute crystal grains of sub micron size in place of the vapor deposition film. For example, the sub micron size minute crystals of boron nitride of cubic system are industrially produced for an application to polishing particles for polishing, and they are moderate in cost, so that this method is practical for forming a large-area electron emission element array.
The structures and manufacturing methods of longitudinal type electron emission elements using such minute particles have been reported/developed in J. Vac. Sci. Technol. B14(1996)2060, U.S. Pat. No. 5,019,003, Japanese Laid-open Patent Application No. Hei-8-241665, Japanese Laid-open Patent Application No. Hei-8-77916, Japanese Laid-open Patent Application No. Hei-10-92294 and Japanese Laid-open Patent Application No. Hei-10-92298.
J. Vac. Sci. Technol. B14(1996)2060 discloses the following technique. According to this technique, an emitter line layer (4002), an insulating film (4001) and a gate film (4003) are deposited on a substrate, and plural holes are formed so as to pierce through the gate film and the insulating film. Further, diamond fine particles (particle diameter of about 1 xcexcm) doped with nitrogen are etched to roughen the surfaces thereof, and then dispersed and pasted in conductive matrix. Thereafter, the paste (4005) thus obtained is filled into the holes on the substrate by a squeegee (4004) to form an electron emission element as shown in FIG. 7. However, the emitter line layer and the gate film of the element thus formed are structurally liable to be short-circuited by a conductive base material and thus it is low in reliability.
The specification of U.S. Pat. No. 5,019,003 discloses an emitter having such a structure that a plurality of fine particles (diameter of 1 xcexcm) are fixed on a substrate 100 by binding agent 101 as shown in FIG. 8. This structure is characterized in that the sharp corners of the fine particles project from the binding agent. Conductive fine particles 201 or insulating fine particles 203 covered by a conductive film 202 may be used as the fine particles. Mo, TiC or the like may be used as the conductive material. The specification of this patent also discloses the arrangement of a gate and anodes for extracting electrons to constitute an electron emission element. In this arrangement, plural fine particle emitters 201 provided on the substrate are covered by an insulating film 409 and gates 401 are arranged on the insulating film 409 as shown in FIG. 9. Further, an insulating film 402 is disposed on the gates 401, and a transparent face plate 404 having a function as an anode electrode and a phosphor layer 403 are disposed on the insulating film 402.
In reality, however, it is not easy to uniformly provide plural fine particles over a large area by the method as disclosed in the above U.S. patent. In order that lots of electrons are emitted, the sharp corners of the fine particles are put face sides up. However, the probability that the sharp corners of the fine particles are put face up is not high, and most of particles do not function as emitters.
In general, the distribution of such parameters as geometric enhancement factors among respective electron emission elements results in much broader distribution of the electric-field/current density characteristics, due to the non-linearity of the field emission. Particularly in a case where an application of the electron emission elements to a display is assumed, the characteristics must be uniform among pixels when the elements are added with gates and fabricated as an array.
Accordingly, it is required that plural electron emission elements constituting pixels should have substantially the same characteristic distribution among the pixels. Therefore, in order to make the characteristic distribution uniform among the pixels, it is necessary that a lot of electron emission elements are contained in each pixel so that the averaging effect can be sufficiently exhibited.
When the size of each pixel is equal to about several hundreds mm square, the maximum number of gate opening portions which can be placed within each pixel is equal to several thousands. However, if the fraction of electron emission elements which do not operate due to unevenness of the arrangement and direction of fine particles is not sufficiently low, the averaging effect is remarkably lowered to the extent that it causes non-uniformity of display which is not allowed in a display.
In addition, the fine particle emitters 201 are located underneath the insulating film in the structure shown in FIG. 9, so that dielectric breakdown is liable to occur in this structure. The thickness of the insulating film must be increased to achieve a sufficient withstanding voltage, and thus the operating voltage rises up.
The Japanese Laid-open Patent Application No. Hei-8-241665 also discloses electrode emission elements using fine particles having the same structure. However, this publication uses as the fine particle material diamond particles activated by hydrogen plasma. The fine particle material of this publication has no specific direction in which electrons are more liable to be emitted, and electron emission occurs from many fine particles. Further, the particle diameter is small (ranging from 10 to 300 nm), so that a large number of fine particles can be placed within an unit area and the averaging effect can be effective. In the structure shown in FIG. 10, a plurality of diamond particles 53 are disposed on a conductive surface 52 provided on a substrate 51, and mask particles 62 are disposed on the diamond particles 53. Thereafter, an insulating film 60 and a gate film 61 are deposited while the mask particles 62 function as masks. This structure still has the problem in dielectric breakdown, and any method of forming a fine particle film uniformly is not disclosed.
In the case of the Japanese Laid-open Patent Application No. Hei-8-77916, an emitter line layer 932 is disposed on a substrate 901 and a conductor 940 containing emitter fine particles 938 is disposed on the emitter line layer 932 through a conductive spacer layer 936 as shown in FIG. 11. The conductor 940 is formed by combining a deposition method such as a sputtering method. An insulating layer 914b and a gate film 907b are provided so as to surround the conductor 940 containing the emitter fine particles.
In this structure, the reliability of the insulating film is improved because the emitter material does not extend into underneath the insulating film unlike the structure described above. However, the deposition process and the patterning process are used to form electron emission elements, and thus the size of the array of the electron emission elements which can be fabricated is limited by the size of a deposition apparatus and an exposing apparatus as in the case of the Spindt method.
Further, according to the method disclosed in this publication, some portions of the insulating film and the gate film which are located above the electron emission portions are removed by using lift-off of the resist when the insulating film and the gate film are disposed. However, it is technically difficult to perform this method because the sum of the film thickness of the insulating film and the gate film is close to 1 xcexcm. Therefore, the yield is low and this method is unsuitable for manufacturing a large-area electron emission element array.
In the case of the Japanese Laid-open Patent Application No. Hei-10-92294, an insulating layer 1003 and a gate electrode line 1004 are disposed on a lower substrate 1001 and a cathode electrode line 1002. Further, an opening portion 1005 is provided and fine particle emitter material is injected from a nozzle into the opening portion 1005 together with high-pressure gas to form a thin film 1007. In this method, however, it is difficult to adjust the amount of fine particles deposited in the opening portion and non-uniformity of display is liable to occur when the electron emission elements thus formed are applied to a display. In addition, the gate and the emitter are liable to be short-circuited in the process of forming the electron emission element.
The common problem in the examples of the electron emission elements using the fine particles described above resides in that when these elements are applied to a display, it is required that the maximum amount of current emitted from the electron emission elements within each pixel cannot be limited. This requirement must be satisfied to suppress occurrence of unevenness in brightness. Accordingly, it is required that an element for limiting the maximum current is installed in each pixel, preferably in each electron emission element. However, any conventional technique described above does not install any structure for limiting the current.
An electron emission source and a display device using the electron emission source disclosed in Japanese Laid-open Patent Application No. Hei-10-92298 are known as a display device using electron emission elements, for example, an extremely thin type display device. The electron emission source and the display device described above will be described with reference to FIGS. 13 and 14.
In the conventional electron emission source, a plurality of stripe-shaped cathode electrode lines 5002 are formed on the surface of a lower substrate 5001 formed of glass material, and a thin film 5007 of material having a small work function is formed on these cathode electrode lines 5002. Further, an insulating film 5003 is formed on the thin film 5007, and a plurality of stripe-shaped gate electrode lines 5004 are formed on the insulating layer 5003 so as to cross the respective cathode electrode lines 5002. The cathode electrode lines 5002 and the gate electrode lines 5004 are formed in a matrix structure. Each cathode electrode line 5002 and each gate electrode line 5004 are connected to control means 5015 to control the driving operation thereof.
In each cross area between the cathode electrode line 5002 and the gate electrode line 5004, a lot of substantially circular holes 5005 are formed so as to pierce through the gate electrode line 5004 and the insulating layer 5003 and extend to the thin film 5007, and the thin film 5007 at the bottom portions of these holes 5005 form a cold cathode.
FIG. 14 shows a display device using this electron emission source. The display device 5020 comprises an electron-emission member having a number of electron emission sources 5012 arranged so as to constitute a display screen, and an upper substrate 5028 disposed so as to be spaced from the electron-emission member at a predetermined interval in the electron emission direction. Stripe-shaped luminescent plates 5029 coated with phosphor which are arranged in parallel to the gate electrode lines 5024 are formed on one surface of the upper substrate which faces the electron emission sources 5012. The gap between the electron emission sources 5012 and the luminescent plate 5029 are kept under vacuum.
Next, the driving operation of the display device 5020 thus fabricated will be described. When the control procedure selects one of the cathode electrode lines 5022 and one of the gate electrode lines 5024 and applies a predetermined voltage across them, electrons are emitted from the electron emission source 5012 at the cross area between them. Further, the electrons are accelerated by a voltage applied across the cathode electrode line 5022 and the upper substrate 5028 serving as the anode, and hit the phosphor on the luminescent plate 5029 to emit visible light, thereby forming an image.
The cross area between the cathode electrode line 5002 and the gate electrode line 5004 constitutes a capacitor using an insulating layer as a dielectric layer. The electrostatic capacitance (parasitic capacitance) Q of the capacitor is represented as follows:
Q=xcex5Oxc3x97xcex5xc3x97A/dxe2x80x83xe2x80x83(1)
xcex50: the permeability of vacuum
xcex5: the permeability of the insulating layer
A: the area of the cross area
d: the thickness of the insulating layer
Therefore, the power W consumed at the capacitance portion under the driving operation is represented as follows:
2W=2pfQV2xe2x80x83xe2x80x83(2)
f: driving frequency
V: driving voltage between gate and emitter
In a conventional light emitting element and a display device using the light emitting element, SiO2 is generally used as the material of the insulating layer 5003. The dielectric constant of the SiO2 thin film formed by CVD or the like is equal to about 4.3, and the parasitic capacitance expressed by the equation (1) is increased to the extent that it cannot be ignored, so that the consumption power of the display device is increased. Further, the thickness of the insulating layer must be increased to suppress the parasitic capacitance within a permissible range, and thus there occurs such a problem that the distance between the gate and the emitter must be increased, resulting in increase of the driving voltage.
As described above, in the conventional electrode emission elements, the structure of the element is simplified by forming the electron emission element of fine particle material, and a high-cost vacuum film forming process can be replaced by a non-vacuum process. However, the conventional techniques have such problems that the reliability of the insulating film cannot be sufficiently ensured from the structural viewpoint and the short-circuiting between the gate wire and the emitter wire occurs.
Furthermore, the conventional techniques have a problem in that the current flowing in each emitter is not limited because the uniformity of display must be kept when the electron emission elements are applied to a display, In addition, there has not been achieved any method which can suppress occurrence of unevenness/defects over a large area without using a vacuum process and uses fine particles uniformly.
Therefore, an object of the present invention is to provide an extremely thin type display device with large picture forming area and long lifetime which can be operated with a low voltage.
According to the present invention, there can be implemented the structure of electron emission elements which has high reliability of insulation between a gate film and an emitter film and has a function of limiting the amount of current emitted from each emitter. Further, a number of the elements can be uniformly manufactured over a large area by using a non-vacuum process.
According to the structure of the electron emission element and the manufacturing method of the electron emission element of the present invention, there can be manufactured an electron emission element array which limits the emission amount of electrons from each emitter and has a uniform characteristic over a large area. Further, electron emission elements using lots of fine particles can be formed over a large area with suppressing occurrence of unevenness/defects by using the non-vacuum process. In addition, the short-circuiting between the gate wire and the emitter wire can be suppressed in the formation process of the electron emission elements.
According to the present invention, there can be achieved a sufficient current limiting effect which is inherent to a resistance layer comprising an insulator and a conductor dispersed in the insulator. Accordingly, when electron emission elements using fine particles are applied to a large-scale display, unevenness of display and occurrence of pixel defects can be effectively suppressed.
Furthermore, by applying the formation method of the elements to an electrophoresis method, the resistance layer and the fine particle layer can be uniformly and selectively deposited on the emitter wire within the gate opening portion, so that the short-circuiting between the gate and the emitter can be suppressed and the reliability of the operation can be remarkably enhanced.
The present invention provides a field emission element comprising: a board; a cathode layer formed on said board; an insulating layer formed on said cathode; a gate layer formed on said insulating layer; a resistance layer formed on said cathode in an opening of said insulating layer and said gate layer, said resistance layer consisting of conductive particles and resistance particles; and an emitter layer formed on said resistance layer, said emitter layer consisting of particles.
The present invention also provides a field emission display comprising; a board; a cathode layer formed on said board; an insulating layer formed on said cathode; a gate layer formed on said insulating layer; a resistance layer formed on said cathode in an opening of said insulating layer and said gate layer, said resistance layer consisting of conductive particles and resistance particles; an emitter layer formed on said resistance layer, said emitter layer consisting of particles; an anode layer opposite said board; and a luminescent layer on said anode layer.
Further, the present invention provides a method for manufacturing a field emission display comprising: forming a cathode layer on a board; forming an insulating layer on said cathode; forming a gate layer on said insulating layer; forming an open in said insulating layer and said gate layer; forming a resistance layer on said cathode in said open by electrophoresis, said resistance layer consisting of conductive particles and resistance particles; and forming an emitter layer on said resistance layer by electrophoresis, said emitter layer consisting of particles.